DESCRIPTION

DB_File is a module which allows Perl programs to make use of the facilities provided by Berkeley DB. If you intend to use this module you should really have a copy of the Berkeley DB manual pages at hand. The interface defined here mirrors the Berkeley DB interface closely.

Please note that this module will only work with version 1.x of Berkeley DB. Once Berkeley DB version 2 is released, DB_File will be upgraded to work with it.

Berkeley DB is a C library which provides a consistent interface to a number of database formats. DB_File provides an interface to all three of the database types currently supported by Berkeley DB.

The file types are:

DB_HASH

This database type allows arbitrary key/value pairs to be stored in data files. This is equivalent to the functionality provided by other hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though, the files created using DB_HASH are not compatible with any of the other packages mentioned.

A default hashing algorithm, which will be adequate for most applications, is built into Berkeley DB. If you do need to use your own hashing algorithm it is possible to write your own in Perl and have DB_File use it instead.

DB_BTREE

The btree format allows arbitrary key/value pairs to be stored in a sorted, balanced binary tree.

As with the DB_HASH format, it is possible to provide a user defined Perl routine to perform the comparison of keys. By default, though, the keys are stored in lexical order.

DB_RECNO

DB_RECNO allows both fixed-length and variable-length flat text files to be manipulated using the same key/value pair interface as in DB_HASH and DB_BTREE. In this case the key will consist of a record (line) number.

Interface to Berkeley DB

DB_File allows access to Berkeley DB files using the tie() mechanism in Perl 5 (for full details, see "tie()" in perlfunc). This facility allows DB_File to access Berkeley DB files using either an associative array (for DB_HASH & DB_BTREE file types) or an ordinary array (for the DB_RECNO file type).

In addition to the tie() interface, it is also possible to access most of the functions provided in the Berkeley DB API directly. See "THE API INTERFACE".

Opening a Berkeley DB Database File

Berkeley DB uses the function dbopen() to open or create a database. Here is the C prototype for dbopen():

The parameter type is an enumeration which specifies which of the 3 interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used. Depending on which of these is actually chosen, the final parameter, openinfo points to a data structure which allows tailoring of the specific interface method.

This interface is handled slightly differently in DB_File. Here is an equivalent call using DB_File:

tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;

The filename, flags and mode parameters are the direct equivalent of their dbopen() counterparts. The final parameter $DB_HASH performs the function of both the type and openinfo parameters in dbopen().

In the example above $DB_HASH is actually a pre-defined reference to a hash object. DB_File has three of these pre-defined references. Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.

The keys allowed in each of these pre-defined references is limited to the names used in the equivalent C structure. So, for example, the $DB_HASH reference will only allow keys called bsize, cachesize, ffactor, hash, lorder and nelem.

To change one of these elements, just assign to it like this:

$DB_HASH->{'cachesize'} = 10000 ;

The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO are usually adequate for most applications. If you do need to create extra instances of these objects, constructors are available for each file type.

Here are examples of the constructors and the valid options available for DB_HASH, DB_BTREE and DB_RECNO respectively.

The values stored in the hashes above are mostly the direct equivalent of their C counterpart. Like their C counterparts, all are set to a default values - that means you don't have to set all of the values when you only want to change one. Here is an example:

A few of the options need extra discussion here. When used, the C equivalent of the keys hash, compare and prefix store pointers to C functions. In DB_File these keys are used to store references to Perl subs. Below are templates for each of the subs:

Note that the like ordinary associative arrays, the order of the keys retrieved is in an apparently random order.

DB_BTREE

The DB_BTREE format is useful when you want to store data in a given order. By default the keys will be stored in lexical order, but as you will see from the example shown in the next section, it is very easy to define your own sorting function.

Changing the BTREE sort order

This script shows how to override the default sorting algorithm that BTREE uses. Instead of using the normal lexical ordering, a case insensitive compare function will be used.

There are a few point to bear in mind if you want to change the ordering in a BTREE database:

The new compare function must be specified when you create the database.

You cannot change the ordering once the database has been created. Thus you must use the same compare function every time you access the database.

Handling Duplicate Keys

The BTREE file type optionally allows a single key to be associated with an arbitrary number of values. This option is enabled by setting the flags element of $DB_BTREE to R_DUP when creating the database.

There are some difficulties in using the tied hash interface if you want to manipulate a BTREE database with duplicate keys. Consider this code:

As you can see 3 records have been successfully created with key Wall - the only thing is, when they are retrieved from the database they seem to have the same value, namely Larry. The problem is caused by the way that the associative array interface works. Basically, when the associative array interface is used to fetch the value associated with a given key, it will only ever retrieve the first value.

Although it may not be immediately obvious from the code above, the associative array interface can be used to write values with duplicate keys, but it cannot be used to read them back from the database.

The way to get around this problem is to use the Berkeley DB API method called seq. This method allows sequential access to key/value pairs. See "THE API INTERFACE" for details of both the seq method and the API in general.

In a scalar context the method returns the number of values associated with the key, $key.

In list context, it returns all the values which match $key. Note that the values will be returned in an apparently random order.

In list context, if the second parameter is present and evaluates TRUE, the method returns an associative array. The keys of the associative array correspond to the values that matched in the BTREE and the values of the array are a count of the number of times that particular value occurred in the BTREE.

Matching Partial Keys

The BTREE interface has a feature which allows partial keys to be matched. This functionality is only available when the seq method is used along with the R_CURSOR flag.

$x->seq($key, $value, R_CURSOR) ;

Here is the relevant quote from the dbopen man page where it defines the use of the R_CURSOR flag with seq:

Note, for the DB_BTREE access method, the returned key is not
necessarily an exact match for the specified key. The returned key
is the smallest key greater than or equal to the specified key,
permitting partial key matches and range searches.

In the example script below, the match sub uses this feature to find and print the first matching key/value pair given a partial key.

DB_RECNO

DB_RECNO provides an interface to flat text files. Both variable and fixed length records are supported.

In order to make RECNO more compatible with Perl the array offset for all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.

As with normal Perl arrays, a RECNO array can be accessed using negative indexes. The index -1 refers to the last element of the array, -2 the second last, and so on. Attempting to access an element before the start of the array will raise a fatal run-time error.

The 'bval' Option

The operation of the bval option warrants some discussion. Here is the definition of bval from the Berkeley DB 1.85 recno manual page:

The delimiting byte to be used to mark the end of a
record for variable-length records, and the pad charac-
ter for fixed-length records. If no value is speci-
fied, newlines (``\n'') are used to mark the end of
variable-length records and fixed-length records are
padded with spaces.

The second sentence is wrong. In actual fact bval will only default to "\n" when the openinfo parameter in dbopen is NULL. If a non-NULL openinfo parameter is used at all, the value that happens to be in bval will be used. That means you always have to specify bval when making use of any of the options in the openinfo parameter. This documentation error will be fixed in the next release of Berkeley DB.

That clarifies the situation with regards Berkeley DB itself. What about DB_File? Well, the behavior defined in the quote above is quite useful, so DB_File conforms it.

That means that you can specify other options (e.g. cachesize) and still have bval default to "\n" for variable length records, and space for fixed length records.

Element 1 Exists with value blue
The last element is yellow
The 2nd last element is blue

Extra Methods

As you can see from the example above, the tied array interface is quite limited. To make the interface more useful, a number of methods are supplied with DB_File to simulate the standard array operations that are not currently implemented in Perl's tied array interface. All these methods are accessed via the object returned from the tie call.

Here are the methods:

$X->push(list) ;

Pushes the elements of list to the end of the array.

$value = $X->pop ;

Removes and returns the last element of the array.

$X->shift

Removes and returns the first element of the array.

$X->unshift(list) ;

Pushes the elements of list to the start of the array.

$X->length

Returns the number of elements in the array.

Another Example

Here is a more complete example that makes use of some of the methods described above. It also makes use of the API interface directly (see "THE API INTERFACE").

ORIGINAL
0: zero
1: one
2: two
3: three
4: four
The last record was [four]
The first record was [zero]
REVERSE
5: last
4: three
3: Newbie
2: one
1: New One
0: first
REVERSE again
5: last
4: three
3: Newbie
2: one
1: New One
0: first

Notice that both times the put method was used the record index was specified using a variable, $i, rather than the literal value itself. This is because put will return the record number of the inserted line via that parameter.

THE API INTERFACE

As well as accessing Berkeley DB using a tied hash or array, it is also possible to make direct use of most of the API functions defined in the Berkeley DB documentation.

To do this you need to store a copy of the object returned from the tie.

$db = tie %hash, "DB_File", "filename" ;

Once you have done that, you can access the Berkeley DB API functions as DB_File methods directly like this:

$db->put($key, $value, R_NOOVERWRITE) ;

Important: If you have saved a copy of the object returned from tie, the underlying database file will not be closed until both the tied variable is untied and all copies of the saved object are destroyed.

All the functions defined in dbopen are available except for close() and dbopen() itself. The DB_File method interface to the supported functions have been implemented to mirror the way Berkeley DB works whenever possible. In particular note that:

The methods return a status value. All return 0 on success. All return -1 to signify an error and set $! to the exact error code. The return code 1 generally (but not always) means that the key specified did not exist in the database.

Other return codes are defined. See below and in the Berkeley DB documentation for details. The Berkeley DB documentation should be used as the definitive source.

Whenever a Berkeley DB function returns data via one of its parameters, the equivalent DB_File method does exactly the same.

If you are careful, it is possible to mix API calls with the tied hash/array interface in the same piece of code. Although only a few of the methods used to implement the tied interface currently make use of the cursor, you should always assume that the cursor has been changed any time the tied hash/array interface is used. As an example, this code will probably not do what you expect:

A return code of 1 means that the requested key was not in the database.

R_CURSOR is the only valid flag at present.

$status = $X->fd ;

Returns the file descriptor for the underlying database.

See "Locking Databases" for an example of how to make use of the fd method to lock your database.

$status = $X->seq($key, $value, $flags) ;

This interface allows sequential retrieval from the database. See dbopen for full details.

Both the $key and $value parameters will be set to the key/value pair read from the database.

The flags parameter is mandatory. The valid flag values are R_CURSOR, R_FIRST, R_LAST, R_NEXT and R_PREV.

$status = $X->sync([$flags]) ;

Flushes any cached buffers to disk.

R_RECNOSYNC is the only valid flag at present.

HINTS AND TIPS

Locking Databases

Concurrent access of a read-write database by several parties requires them all to use some kind of locking. Here's an example of Tom's that uses the fd method to get the file descriptor, and then a careful open() to give something Perl will flock() for you. Run this repeatedly in the background to watch the locks granted in proper order.

Sharing Databases With C Applications

There is no technical reason why a Berkeley DB database cannot be shared by both a Perl and a C application.

The vast majority of problems that are reported in this area boil down to the fact that C strings are NULL terminated, whilst Perl strings are not.

Here is a real example. Netscape 2.0 keeps a record of the locations you visit along with the time you last visited them in a DB_HASH database. This is usually stored in the file ~/.netscape/history.db. The key field in the database is the location string and the value field is the time the location was last visited stored as a 4 byte binary value.

If you haven't already guessed, the location string is stored with a terminating NULL. This means you need to be careful when accessing the database.

Here is a snippet of code that is loosely based on Tom Christiansen's ggh script (available from your nearest CPAN archive in authors/id/TOMC/scripts/nshist.gz).

Although the error message above refers to the second tie() statement in the script, the source of the problem is really with the untie() statement that precedes it.

Having read perltie you will probably have already guessed that the error is caused by the extra copy of the tied object stored in $X. If you haven't, then the problem boils down to the fact that the DB_File destructor, DESTROY, will not be called until all references to the tied object are destroyed. Both the tied variable, %x, and $X above hold a reference to the object. The call to untie() will destroy the first, but $X still holds a valid reference, so the destructor will not get called and the database file tst.fil will remain open. The fact that Berkeley DB then reports the attempt to open a database that is alreday open via the catch-all "Invalid argument" doesn't help.

If you run the script with the -w flag the error message becomes:

untie attempted while 1 inner references still exist at bad.file line 12.
Cannot tie second time: Invalid argument at bad.file line 14.

which pinpoints the real problem. Finally the script can now be modified to fix the original problem by destroying the API object before the untie:

COMMON QUESTIONS

Why is there Perl source in my database?

If you look at the contents of a database file created by DB_File, there can sometimes be part of a Perl script included in it.

This happens because Berkeley DB uses dynamic memory to allocate buffers which will subsequently be written to the database file. Being dynamic, the memory could have been used for anything before DB malloced it. As Berkeley DB doesn't clear the memory once it has been allocated, the unused portions will contain random junk. In the case where a Perl script gets written to the database, the random junk will correspond to an area of dynamic memory that happened to be used during the compilation of the script.

Unless you don't like the possibility of there being part of your Perl scripts embedded in a database file, this is nothing to worry about.

How do I store complex data structures with DB_File?

Although DB_File cannot do this directly, there is a module which can layer transparently over DB_File to accomplish this feat.

Check out the MLDBM module, available on CPAN in the directory modules/by-module/MLDBM.

What does "Invalid Argument" mean?

You will get this error message when one of the parameters in the tie call is wrong. Unfortunately there are quite a few parameters to get wrong, so it can be difficult to figure out which one it is.

Here are a couple of possibilities:

Attempting to reopen a database without closing it.

Using the O_WRONLY flag.

What does "Bareword 'DB_File' not allowed" mean?

You will encounter this particular error message when you have the strict 'subs' pragma (or the full strict pragma) in your script. Consider this script:

To get around the error, place the word DB_File in either single or double quotes, like this:

tie %x, "DB_File", "filename" ;

Although it might seem like a real pain, it is really worth the effort of having a use strict in all your scripts.

HISTORY

0.1

First Release.

0.2

When DB_File is opening a database file it no longer terminates the process if dbopen returned an error. This allows file protection errors to be caught at run time. Thanks to Judith Grass <grass@cybercash.com> for spotting the bug.

0.3

Added prototype support for multiple btree compare callbacks.

1.0

DB_File has been in use for over a year. To reflect that, the version number has been incremented to 1.0.

Added complete support for multiple concurrent callbacks.

Using the push method on an empty list didn't work properly. This has been fixed.

1.01

Fixed a core dump problem with SunOS.

The return value from TIEHASH wasn't set to NULL when dbopen returned an error.

1.02

Merged OS/2 specific code into DB_File.xs

Removed some redundant code in DB_File.xs.

Documentation update.

Allow negative subscripts with RECNO interface.

Changed the default flags from O_RDWR to O_CREAT|O_RDWR.

The example code which showed how to lock a database needed a call to sync added. Without it the resultant database file was empty.

Fixed fd method so that it still returns -1 for in-memory files when db 1.86 is used.

1.11

Documented the untie gotcha.

1.12

Documented the incompatibility with version 2 of Berkeley DB.

1.13

Minor changes to DB_FIle.xs and DB_File.pm

1.14

Made it illegal to tie an associative array to a RECNO database and an ordinary array to a HASH or BTREE database.

1.15

Patch from Gisle Aas <gisle@aas.no> to suppress "use of undefined value" warning with db_get and db_seq.

Patch from Gisle Aas <gisle@aas.no> to make DB_File export only the O_* constants from Fcntl.

Removed the DESTROY method from the DB_File::HASHINFO module.

Previously DB_File hard-wired the class name of any object that it created to "DB_File". This makes sub-classing difficult. Now DB_File creats objects in the namespace of the package it has been inherited into.

BUGS

Some older versions of Berkeley DB had problems with fixed length records using the RECNO file format. The newest version at the time of writing was 1.85 - this seems to have fixed the problems with RECNO.

I am sure there are bugs in the code. If you do find any, or can suggest any enhancements, I would welcome your comments.

AVAILABILITY

DB_File comes with the standard Perl source distribution. Look in the directory ext/DB_File.

This version of DB_File will only work with version 1.x of Berkeley DB. It is not yet compatible with version 2.

Version 1 of Berkeley DB is available at your nearest CPAN archive (see "CPAN" in perlmod for a list) in src/misc/db.1.85.tar.gz, or via the host ftp.cs.berkeley.edu in /ucb/4bsd/db.tar.gz. Alternatively, check out the Berkeley DB home page at http://www.bostic.com/db. It is not under the GPL.

If you are running IRIX, then get Berkeley DB from http://reality.sgi.com/ariel. It has the patches necessary to compile properly on IRIX 5.3.

As of January 1997, version 1.86 of Berkeley DB is available from the Berkeley DB home page. Although this release does fix a number of bugs that were present in 1.85 you should be aware of the following information (taken from the Berkeley DB home page) before you consider using it:

DB version 1.86 includes a new implementation of the hash access
method that fixes a variety of hashing problems found in DB version
1.85. We are making it available as an interim solution until DB
2.0 is available.
PLEASE NOTE: the underlying file format for the hash access method
changed between version 1.85 and version 1.86, so you will have to
dump and reload all of your databases to convert from version 1.85
to version 1.86. If you do not absolutely require the fixes from
version 1.86, we strongly urge you to wait until DB 2.0 is released
before upgrading from 1.85.